FIELD OF THE INVENTION
The invention relates to a nuclear-fuel sintered pellet containing UO.sub.2, (U, Pu)O.sub.2, (U, Th)O.sub.2, (U, RE)O.sub.2, (U, Pu, Th)O.sub.2, (U, Pu, RE)O.sub.2, (U, Th, RE)O.sub.2 or (U, Pu, Th, RE)O.sub.2, wherein RE=rare earth. The invention also relates to a nuclear-reactor fuel assembly including a fuel rod having a cladding tube and such a uranium-containing nuclear-fuel sintered pellet in the cladding tube. The invention additionally relates to a method for treating such a uranium-containing nuclear-fuel sintered pellet.
Published European Patent Application 0 239 843 A1, corresponding to U.S. Pat. No. 4,774,051, discloses a nuclear-fuel sintered pellet made of UO.sub.2, (U, Pu)O.sub.2 or (U, Th)O.sub.2. Boron is incorporated as a neutron poison in the chemical compound form UB.sub.x, wherein x=2; 4 and/or 12 and/or B.sub.4 C, in a sinter matrix of that nuclear-fuel sintered pellet. That known nuclear-fuel sintered pellet is obtained by producing a mixture of uranium oxide powder or uranium mixed oxide powder with uranium boride powder or boron carbide powder and pressing it to form pellets which are subsequently sintered in a sintering furnace under a reducing sintering atmosphere to form nuclear-fuel sintered pellets. In those nuclear-fuel sintered pellets, the boron is thereby uniformly distributed throughout the sinter matrix.
From the neutron physics point of view, boron in uranium-containing nuclear-fuel sintered pellets is a burnable neutron absorber which loses its property as an absorber for thermal neutrons after those nuclear-fuel sintered pellets have been used in a nuclear reactor for a certain period of time.
Nuclear-reactor fuel assemblies having fuel rods that contain uranium-containing nuclear-fuel sintered pellets are used in a nuclear reactor, for example, during four sequential fuel assembly cycles, generally being of equal durations. At the end of a fuel assembly cycle, some of the nuclear-reactor fuel assemblies in the nuclear reactor are in each case replaced by fresh, unirradiated nuclear-reactor fuel assemblies.
The fresh, unirradiated nuclear-reactor fuel assemblies would cause a comparatively high reactivity in the nuclear reactor relative to the nuclear-reactor fuel assemblies that are already irradiated. However, the boron in the nuclear-fuel sintered pellets of those fresh, unirradiated nuclear-reactor fuel assemblies at first moderates the reactivity due to those nuclear-reactor fuel assemblies by initially absorbing thermal neutrons.
The nuclear fuel in fresh and unirradiated nuclear-reactor fuel assemblies gradually burns out in the nuclear reactor through nuclear decay, but a burnable neutron absorber that is present in that nuclear fuel simultaneously burns out gradually due to the physical effects of neutrons, so that finally, that neutron absorber absorbs no thermal neutrons or only very few. In that way, even unirradiated nuclear-reactor fuel assemblies being newly loaded into the nuclear reactor may cause approximately the same reactivity in the nuclear reactor during their entire residence time in the nuclear reactor, as the nuclear-reactor fuel assemblies which have already spent a fuel assembly cycle in the nuclear reactor.
Boron is advantageously used as a neutron absorber in a nuclear fuel as compared to other burnable neutron absorbers such as rare earths if the fuel assembly cycles are comparatively long, i.e., for example, longer than 12 months, since accumulation of heat in the nuclear fuel is avoided with boron.